The invention relates to compositions comprising a combination of a drug and a concentration-enhancing polymer that enhances the concentration of the drug in a use environment relative to control compositions that are free from the concentration-enhancing polymer.
Low-solubility drugs often show poor bioavailability or irregular absorption, the degree of irregularity being affected by factors such as dose level, fed state of the patient, and form of the drug. Increasing the bioavailability of low-solubility drugs has been the subject of much research. Increasing bioavailability hinges on improving the concentration of the drug in solution to improve absorption.
It is known that many low-solubility drugs can be formulated so as to increase the maximum concentration of the drug that will dissolve in an aqueous solution in in vitro tests. When such a drug in a solubility-improved form is initially dissolved in an environment of use, such as in gastric fluid, the solubility-improved form of the drug initially provides a higher concentration of dissolved drug in the environment of use relative to other forms of the drug and relative to the equilibrium concentration of the drug. In addition, it has been shown that when such forms are tested in vivo they can enhance the relative bioavailability of the drug, presumably by enhancing, at least temporarily, the concentration of dissolved drug present in the gastrointestinal (GI) tract. However, as discussed below, the higher concentration obtained is often only temporary, with the solubility-improved drug form quickly converting to the low-solubility form after delivery to a use environment.
For example, it is known that some low-solubility drugs may be formulated in highly soluble salt forms that provide temporary improvements in the concentration of the drug in a use environment relative to another salt form of the drug. An example of such a drug is sertraline, which in the lactate salt form has a higher aqueous solubility at pH 3 than the HCl salt form. However, when a high-solubility salt form such as sertraline lactate is dosed to an aqueous solution (either in vitro or in vivo) that has both high levels of chloride present as well as buffers to control the pH, the enhanced solubility of the sertraline lactate is either short lived or not achieved at all since the sertraline can quickly convert to crystalline or amorphous HCl or free base forms that have lower solubility than sertraline lactate.
Another drug form known to provide, at least temporarily, increased concentrations in solution of low-solubility drugs consists of drug in a hydrate or solvate crystalline form of the drug. Such forms often have higher aqueous solubility relative to the lowest solubility crystalline form and, therefore, provide higher concentrations of drug.
It is known that some drugs are capable of forming more than one crystal structure, despite having identical chemical compositions. (This is in contrast to salt forms, solvates, or hydrates that have varying chemical compositions.) These various crystal structures are often referred to as polymorphs. Polymorphs comprise another drug form that temporarily provides increased concentrations in solution. Some polymorphs, also referred to herein as “high-energy crystalline forms,” have higher aqueous solubility and therefore can provide enhanced aqueous concentration of the drug relative to other crystal structures and relative to the equilibrium concentration.
It is also well known that the amorphous form of a low-solubility drug that is capable of existing in either the crystalline or amorphous form may also temporarily provide a greater aqueous concentration of drug relative to the equilibrium concentration of drug in a use environment. It is believed that the amorphous form of the drug dissolves more rapidly than the crystalline form, often dissolving faster than the drug can precipitate from solution. As a result, the amorphous form may temporarily provide a greater-than equilibrium concentration of drug.
Another method that can temporarily provide a greater than equilibrium drug concentration is to include a solubilizing agent in the drug form. Such solubilizing agents promote the aqueous solubility of the drug. An example of the use of a solubilizing agent with a drug to increase aqueous solubility is the use of solubilizing agents with sertraline. As disclosed in commonly assigned PCT Application No. 99/01120, now abandoned, when sertraline is codissolved in aqueous solution with a solubilizing agent, for example, citric acid, the solubility of sertraline is dramatically increased. As mentioned above, when sertraline HCl is dosed along with citric acid to a chloride-containing buffer solution or the GI tract, the maximum sertraline concentration achieved can exceed the solubility of sertraline HCl. This concentration enhancement is thought to be partly due to a locally lower pH in the use environment due to the presence of the citric acid and partly due to the presence of citrate counter ions, as sertraline citrate is more soluble than sertraline chloride. However, the enhanced concentration is typically short-lived as sertraline quickly converts to a low-solubility form which could be, depending on the use environment, the solid crystalline or amorphous HCl salt and/or crystalline or amorphous free base.
Yet another technique for temporarily achieving a greater than equilibrium concentration of drug in a use environment is to formulate the drug as an aqueous or organic solution. For example, drug can be dissolved in polyethylene glycol (PEG) or an aqueous solution of PEG to which an acid or base may be added or the drug may be dissolved in an aqueous solution of an acid or base. Alternatively, the drug can be dissolved in a pharmaceutically acceptable organic liquid such as glycerol, mono-, di-, or triglycerides, fats or oils.
While these solubility-improved drug forms show initially enhanced concentration of the drug in a use environment, nevertheless the improved concentration is often short-lived. Typically, the initially enhanced drug concentration is only temporary and quickly returns to the lower equilibrium concentration. For example, while a particular salt form of a basic drug may show improved initial aqueous concentration, the drug often rapidly converts in gastric fluid to another salt form (typically the HCl salt form) that has a much lower equilibrium concentration. In other cases, the drug maintains acceptable solubility in the low pH gastric solution, but precipitates typically as the free-base form of the drug upon passing into the small intestine where the pH is high, typically from 4.4 to 7.5. Since drug absorption occurs primarily in the intestines, such drug dosage forms that do not sustain high concentration of the drug in an intestinal solution typically yield only minor improvements in bioavailability. Likewise, a high-solubility salt form of an acidic drug can rapidly convert to another salt form that has a much lower equilibrium concentration. Similar effects are observed even for high solubility salt forms of zwitterionic drugs. Similarly, once the high-energy crystalline form of a drug (e.g., a polymorph) dissolves, the drug often rapidly precipitates or crystallizes from solution as it changes to a lower energy crystalline form or an amorphous form with lower solubility which causes concentration of dissolved drug to approach a lower equilibrium concentration.
One approach to increase the bioavailability of low-solubility drugs has involved forming amorphous dispersions of drugs with polymers. Examples of attempts to increase drug concentration by foaming a dispersion of the drug with a polymer include Lahr et al., U.S. Pat. No. 5,368,864, Kanikanti et al., U.S. Pat. No. 5,707,655, and Nakamichi et al., U.S. Pat. No. 5,456,923.
However, creating an amorphous dispersion of a drug and polymer(s) does have some drawbacks. There is a risk that in the process of creating the dispersion, the drug will be altered. For example, some drugs may degrade at the elevated temperatures used to form some dispersions. Some processes use organic solvents which must be thoroughly removed to avoid drug degradation. Solvents must be chosen which dissolve both the drug and the polymer. The process of forming such dispersions is also time-consuming and expensive. In addition, the dispersions may in some cases be unstable and may either chemically degrade over time at moderate temperature and humidity levels or the drug may convert to a lower energy and lower solubility amorphous or crystalline form.
Increasing drug solubilization by using combinations of drug and polymer has also been described. For example, Martin et al., U.S. Pat. No. 4,344,934 mixed poorly-soluble drugs with polymers such as hydroxypropyl methyl cellulose (HPMC) and added an aqueous surfactant solution to the drug-polymer mixture. While this results in improved dissolution, there is only slight enhancement of drug concentration relative to the equilibrium concentration. Piergiorgio et al., U.S. Pat. No. 4,880,623 used solvent processing to co-precipitate nifedipine with PEG and adsorbed this onto polymers such as HPMC, or onto other excipients. While increased drug bioavailability was observed, no comparison was made between different drug forms. Uedo et al., U.S. Pat. No. 5,093,372 mixed the sparingly-soluble drug exifone with polymers such as HPMC to increase bioavailability. However, this did not result in any enhanced drug concentration of the drug/polymer mixture relative to the bulk crystalline form of the drug.
In addition, combining drugs with solubilizing polymers is not universally available to improve bioavailability for all low-solubility drugs. Drug solubilization is usually highly dependent upon the chemical structure and physical properties of the specific drug and therefore the particular polymer, if any, that may prove to solubilize the drug varies from drug to drug. It is often difficult and time-consuming to select polymers which achieve improved solubilization, since the drug-polymer interaction is poorly understood. Often, addition of polymers simply speeds dissolution of the drug, as opposed to providing enhanced concentration.
Usui, et al., Inhibitory Effects of Water-soluble Polymers on Precipitation of RS-8359, Int'l J. of Pharmaceutics 154 (1997) 59-66, discloses the use of three polymers, namely hydroxy propyl methyl cellulose, hydroxy propyl cellulose, and polyvinylpyrrolidone to inhibit precipitation of the low-solubility drug RS-8359. The drug and polymer were dissolved in a mixture of 0.5 N HCl and methanol, and then added to a phosphate buffer solution. Usui et al. observed that the particular polymers inhibited crystallization of the drug.
Accordingly, what is still needed is a composition comprising a drug that provides enhanced concentration of the drug in aqueous solution relative to the equilibrium concentration of the drug, that maintains the concentration of the drug in such a solution over time or at least reduces the rate at which the drug concentration decreases from the enhanced concentration to the equilibrium concentration, that may be prepared using processes that will not alter or degrade the drug, that may be prepared without relying on solvent processing, that is stable under typical storage conditions, that may be easily and cheaply prepared and that ultimately enhances the bioavailability of poorly soluble drugs. These needs and others that will become apparent to one of ordinary skill are met by the present invention, which is summarized and described in detail below.